Steam turbines are machines that are used to generate mechanical (rotational motion) power from the pressure energy of steam. Thus, a steam turbine's primary components are blades, which are designed to produce maximum rotational energy by directing the flow of steam along their surfaces. To maximize turbine efficiency, the steam is expanded (and thereby reduced in pressure) as it flows through the turbine, generating work in a number of stages of the turbine.
In some steam turbine designs, steam from the high pressure end packing is routed between the inner and outer shells of the turbine to provide sealing steam to the low pressure end packing of the turbine. Some of this sealing steam is allowed to re-enter the main steam flow after the last stage of the steam turbine. This steam re-enters the main steam flow before the pressure and temperature of the main steam flow can be measured. This causes the measured efficiency of the steam turbine to be lower than if there was no sealing steam entering the main steam flow.
The problem with current testing of steam turbine efficiency occurs when the measured steam turbine efficiency is less than the expected value. There are two possible causes for this situation. The first is that the internal leakage flow is higher than design, causing an increase in the turbine exhaust enthalpy. The second is that the steam path efficiency is lower than the design value. The current test procedure cannot determine which caused the decrease in performance.